1. Field of the Invention
The present invention pertains to a cardioverter defibrillator, and more particularly, to an improved dual battery power system for use with an implantable cardioverter defibrillator.
2. Background of the Invention
Implantable cardioverter defibrillators have several unique battery requirements, as compared to other implantable products. An implantable cardioverter defibrillator demands a battery with the following general characteristics: very high reliability, highest possible energy density (i.e., small size), extremely low self-discharge rating (i.e., long shelf life), very high current capability, high operating voltage, and high sealability (i.e., no gas or liquid venting).
Some of these parameters have some measure of mutual exclusivity, making it difficult to optimize the battery or electronics without making compromises to the design of the implantable device. In its monitoring mode, the implantable cardioverter defibrillator requires the battery to deliver continuous currents in the range of only 10-30 .mu.A, while in its defibrillation mode, the same battery must deliver currents in the range of one to two amps, some five orders of magnitude greater than the current required for the monitoring mode.
Presently, all manufactured implantable cardioverter defibrillators use a single battery system to power the implantable device. The longevity of an implanted cardioverter defibrillator with a single battery configuration and the number of shocks the defibrillator is capable of delivering are strictly dependant on the remaining battery capacity at any given time. As the device ages, its ability to deliver an adequate number of defibrillating shocks declines as the battery is depleted by the monitoring electronics. Similarly, if a patient receives a large number of shocks soon after implant, the remaining monitoring life is reduced. Thus, it is difficult to assess the condition of the battery and its remaining useful life after it has been in use for a period of time.
A further disadvantage of the single battery configuration is that the ideal voltage requirements for the monitoring and output functions are opposite. For the monitoring function, it is desirable to use the lowest possible voltage that the circuits can operate reliably with in order to conserve energy. This is typically in the order of 1.5-3.0 V. On the other hand, the output circuit works most efficiently with the highest possible battery voltage in order to produce firing voltages of up to about 750 V.
All existing manufactured implantable cardioverter defibrillators have compromised between these two demands by using a single battery system or configuration which is typically comprised of two lithium silver vanadium pentoxide cells electrically connected in series to produce an output battery voltage of about 6 V. The battery voltage must be elevated via an inverter circuit to the firing voltage of about 750 V. The net result is that power is wasted in both the monitoring and output circuits because the monitoring circuit which requires only 2-3 V must operate from a relatively high 6 V source, and the output circuit whose efficiency is a function of the supply voltage must operate from the relatively low 6 V source.
At least two previous development attempts have been made to avoid some of the problems inherent in using a single battery system configuration for an implantable cardioverter defibrillator. In the Medtronic Model 2315, lithium thionyl chloride batteries were employed for the high-voltage charging circuit and lithium manganese dioxide batteries were used for the remaining low voltage circuitry. Similarly, the Telectronics Model 4201 initially tried to employ separate batteries for the low voltage circuits (lithium iodine) and high-voltage circuits (lithium silver vanadium pentoxide). Troop, P. J., "Implantable Cardioverters and Defibrillators", Current Problems in Cardiology, Vol. XIV, No. 12, (December 1989), pp. 703-04. Unfortunately, neither of these devices resulted in practical, manufactured implantable cardioverter defibrillators and the dual battery approach was abandoned in both cases.
While single battery systems have proved workable for implantable cardioverter defibrillators, the use of a single battery system necessarily involves a compromise between the ideal power supplies which would otherwise be used for the various types of circuitry within the implantable cardioverter defibrillator. Accordingly, it would be desirable to provide for an improved dual battery power system for an implantable cardioverter defibrillator which avoids the need for the compromises required of single battery systems, and which overcomes the problems of earlier attempts at dual battery systems.